1. Field of the Invention
The present invention relates to a foil-decorating injection molding machine which uses a decorative sheet carrying patterns, characters, etc. to produce a molding decorated by the patterns, characters, etc. of the decorative sheet, and to a foil-decorating injection molding method carried out, using the foil-decorating injection molding machine.
2. Description of the Related Art
A generally known foil-decorating injection molding method, such as disclosed in U.S. Pat. No. 4,639,341 to Hanamoto et al., which forms a molding by injection molding and attaches a web-fed or sheet-fed decorative sheet to the molding simultaneously, comprises steps of feeding the decorative sheet into a space between a male mold and a female mold of an injection mold, preheating the decorative sheet by a hot plate or the like for softening, closely attaching the softened decorative sheet to the cavity surface of the female mold by vacuum forming and/or pressure forming, joining together the male mold and the female mold to close the injection mold, and injecting a molten resin through the male mold into the cavity to injection-mold the molding.
A laminated decorative sheet or a transfer decorative sheet is used depending on the type of a product to be produced. If a laminated decorative sheet is used, the laminated decorative sheet is attached integrally to the surface of an injection-molded molding to form a decorative layer. If a transfer decorative sheet, which has a substrate and a decorative transfer layer formed on the substrate, is used, the transfer decorative sheet is attached to the surface of a molding, and then the substrate is removed from the molding so that only the decorative transfer layer remains on the surface of the molding.
A foil-decorating injection molding machine provided with an injection mold having a male mold and a female mold forming a cavity is used for simultaneous injection molding and foil decoration. FIG. 8 shows a female mold of an injection mold 50 and associated parts employed in a typical, conventional foil-decorating injection molding machine, and FIG. 9 shows the injection mold 50 of FIG. 8 in a closed state. The injection mold 50 is designed for forming a molding 1 (FIG. 5) of a height H of 50 mm, having a large end of a diameter D of 50 mm and a small end of a diameter E of 44 mm, and having an outer surface coated with a laminated decorative sheet S. The injection mold 50 has a male mold 12 and a female mold 20. The male mold 12 is provided with a runner 14 in its central portion. A molten resin is injected through the runner 14 into a cavity formed by the male mold 12 and the female mold 20 by an injection unit. The female mold 20 has a cavity surface 22 corresponding to the outer surface of the molding 1. A ring-shaped sheet holding member 24 is supported at its four corners by a sliding rod for movement relative to the female mold 20 to hold a decorative sheet S fixedly on a parting surface of the female mold 20 merging into the cavity surface 22. The sheet holding member 24 is driven for movement by a driving mechanism, such as a hydraulic cylinder actuator, installed on the bottom of the female mold 20. An elastic O-ring 27 is fitted in an endless groove formed in the parting surface 40 of the female mold 20 so as to correspond to the sheet holding member 24 for hermetic sealing. The female mold 20 is provided with suction holes 16 opening in the cavity surface 22 to attract the decorative sheet S closely to the cavity surface 22 by suction for preforming. The suction holes 16 are connected to a suction pipe 19 which in turn is connected to a vacuum source. If necessary, the male mold 12 may be provided with a compressed air supply holes connected to a compressed air source to blow compressed air against the decorative sheet S for preforming.
In the operation of the foil-decorating injection molding machine provided with the injection mold 50 thus constructed, a decorative sheet S for the next injection molding cycle is placed on the parting surface 40 of the female mold 20 behind the sheet holding member 24, the decorative sheet S is held fixedly on the parting surface 40 of the female mold 20 by the sheet holding member 24, the decorative sheet S is heated by a hot plate or the like for hot softening, a suction is applied through the suction holes 16 to the decorative sheet S in order that the decorative sheet S is extended along and in close contact with the cavity surface of the female mold 20 for performing, the female mold 20 is advanced toward the male mold 12 to close the injection mold 50 so that the decorative sheet S is held between the parting surface 40 of the female mold 20 and the parting surface 30 of the male mold 12, a molten resin is injected through the runner 14 of the male mold 12 into the cavity 22 defined by the male mold 12 and the female mold 20 for injection molding, the injection mold 50 is cooled for a predetermined time, the female mold 20 is moved away from the male mold 12 to open the injection mold 50, and then a decorated molding 1 decorated by the decorative sheet S is removed from the female mold 20.
In the conventional foil-decorating injection molding machine provided with the injection mold 50, the decorative sheet S is expanded locally and irregularly because the female mold 20 has a flat parting surface 40; that is, a decorating portion Sb (FIG. 8), i.e., a portion to be attached to a molding, of the decorative sheet S corresponding to the cavity 22 is expanded far more greatly than a portion Sa of the decorative sheet S lying on the parting surface 40, because the portion Sa, when heated for softening, cools off and hardens quickly due to the dissipation of heat given thereto by heat transfer from the portion Sa to the female mold 20 by conduction, and the expansion of the portion Sa is obstructed by the frictional resistance of the parting surface 40 of the female mold 20. On the other hand, the portion Sb floating over the cavity surface 22 is not restrained from expansion and hence the portion Sb is expanded locally and concentratedly. When the cavity surface 22 has a shape resembling the shape of a die surface for deep drawing, the decorative sheet S is liable to be broken or creased to cause faulty molding. More specifically, a strain is expressed by a change in length per unit undistorted length, i.e., a strain .DELTA.L/L where .DELTA.L is a change in length of an object of an undistorted length L. Therefore, even if the change .DELTA.L in length is the same, an object having a shorter length L has a greater strain .DELTA.L/L and an object having a greater length L has a smaller strain .DELTA.L/L.